I have never seen any testing of a 5 yr or older helmet proving that it doesn't provide protection anymore.
So all I conclude at this time is that they want to sell helmets.
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as a late comer to this topic, thanks to all past posters that have contributed to the discussion. Lots of good points are made from different perspectives. In the end, it is up to each person, each rider to follow his or her own feelings as to how to protect themselves in the "most prudent or statistically sound" manner......
So why am I dredging this up..... it's old news..... OK, I guess I am bored.
As a person who totally thinks the "5 year rule" is the biggest crock of horse pucky I ever heard...... I will also be the first to say "knock yourself out" if you want to drop 2, 3, 4, 5, 600 $ on a new lid every 5 years ( or maybe less for you hardcore " I ride every day of the year" types) . It is fine with me if that makes you feel safer.
Personally, and I am basing this on my personal riding habits, the amount of time I spend actually using a helmet any given year, and my storage and care habits of the helmet when not being used, that it would be an absolute total waste of my money to follow the so called "rule of thumb"...... but then, I waste money every day...... ( Just not to the tune of $500)

If helmet manufacturer's had any real evidence via testing , etc..... don't you think it would be splattered all over the media to help sell even more helmets? Damn right it would be. In fact, I would venture to say there would be a lobbyist in Washington making sure that our federal government in its wisdom and glory, was regulating the use of and overseeing via legislation that we all were "properly" protecting ourselves.......
I'm not saying they aren't concerned about your and my safety, ........ I'm sure they are, they had better be, right ? but I'm quite sure they are doing what they do for the money in the end ( don't we all ? ) ....... and heck I really can't blame them.... when all you have to do is insinuate and not really prove something..... well, why not do it ? If its a lie, it is a "white lie" and the only thing it's costing anyone is a little money.... so what ....... 1-Sunlight, UV .... breaking down a shell ? say what ? How many helmets are bare on the exterior, no paint, no decal , etc....? How does this UV get to the inside to "break it down" . ? More horse pucky.....2-the liners.... well heck yes they can age, start to dry rot, stink , etc.... if you leave it in the heat and humidty 365 days a year, you're damn skippy it will stink, ....break down...etc..... so....learn to take better care of the equipment..... or buy a new liner once in a while.. ....... , is it that hard.?3-new construction is better ? So the helmet I had 10 years ago was a piece of crap, and I am just lucky I never needed it to do it's job ? Bull crap...... OK, improvements in, and usage of new materials, reduced weight, better ventilation, etc.... I am not arguing that helmets haven't progressed tremendously even over the past 5 years .... It's nothing short of amazing.... but to say that my 2003 Shoei RF1000 or whatever the hell it is, is no longer any good just because it is 11 years old ? It looks much like the day I bought it..... it has been stored inside my home ( climate controlled ) when not used riding, and I'd trust it today..... even though I have also picked up an Arai XD3 ( used BTW), an HJC off road style helmet which I have no idea the model, and last year, I bought an open face helmet at a Harley dealership ( oh my god, I said the "O" word didn't I, and in the same sentence with the "H" word !!!!? ) with a flip shield, the tinted visor that flips down from inside ..... great ventilation, and it is my favorite helmet out of all of them...... so go figure.
Hey so everyone ride safe, do what makes you feel OK.... that's what I intend to do..... but right now I am going to sleep..... way too late for posting to a 3 year old thread....anyway.... right?

__________________"Life is what happens, when you're busy making other plans...."